The invention relates to a method of recording marks representing data in an information layer of a record carrier by irradiating the information layer with a pulsed radiation beam, each mark being written by a sequence of pulses, the recorded marks being erasable by irradiating the information layer with a radiation beam having an erase power level (e), said information layer having a phase which is reversibly changeable between a crystal phase and an amorphous phase and said sequence of pulses comprising write pulses having a first write power level (w1), a bias power level (b) between the write pulses, and a trailing power level (t) after a last write pulse of a sequence.
The invention also relates to an optical recording device for recording data in the form of marks in an information layer of a record carrier by irradiating the information layer with a pulsed radiation beam, said information layer having a phase which is reversibly changeable between a crystal phase and an amorphous phase, the device comprising a radiation source providing the radiation beam and a control unit for controlling the power of the radiation beam, the control unit providing a sequence of write pulses for writing a mark and controlling the power of the radiation beam such that it has a first write power level (w1) during the write pulses in a sequence of write pulses, a bias power level (b) between the write pulses in a sequence of write pulses, a trailing power level (t) after a last write pulse of a sequence of write pulses, and an erase power level (e) between sequences of write pulses.
A recording method and a device as defined in the opening paragraphs are known from the international patent application WO 97/30440 (publication date 21 Aug. 1997). A mark is written by a sequence of write pulses. Previously written marks, if any, between the marks being written are erased by applying an erase power level in between the sequences of write pulses. The known sequence has a bias power level between the write pulses and a return to the bias power level after the last write pulse of a sequence of write pulses before rising to an erase power level.
International patent application WO 98/36411 (publication date 20 Aug. 1998) discloses an improved method, which method has the feature that the sequence of write pulses has a bias power level between the write pulses and a cooling power level after the last write pulse of a sequence of write pulses before rising to the erase power level, the cooling power level being in a range between the bias power level and the erase power level.
A change of the bias power level in the known methods affects both the leading-edge jitter and the trailing-edge jitter. The jitter is the standard deviation of the time differences between level transitions in the digitized read signal and the corresponding transitions in a clock signal, the time differences being normalized by the duration of one period of said clock. The last mentioned method, described in WO 98/36411, introduces an additional degree of freedom by making the cooling power level during the cooling period, i.e. the period following the last write pulse in a sequence of write pulses and preceding the rise to the erase power level, independent of the bias power level. This allows the trailing-edge jitter to be influenced almost independently of the leading-edge jitter.
The known methods are suitable for direct-overwrite on a record carrier, i.e. writing information to be recorded in the information layer of the record carrier and at the same time erasing information previously written in the information layer.
It is a disadvantage of the known methods, especially when the marks are written at high recording speeds, that they do not allow a sufficient reduction of the jitter in the read signal obtained from reading marks written by using the known methods.
It is an object of the invention to provide a method of recording marks of the kind described in the opening paragraph having a further reduced jitter.
This object is achieved when the method of the preamble is characterized in that the trailing power level (t) is higher than the erase power level (e) and lower than the first write power level (w1).
The method according to the invention replaces the cooling power level in the known methods, which is lower than the erase power level (e), by a trailing power level (t) higher than the erase power level (e). Instead of cooling the information layer after a sequence of write pulses, a more or less gradual transition from the write power level of the last write pulse to the erase power level is introduced, resulting in, especially, the trailing-edge jitter of the marks being smaller than the trailing-edge jitter of the marks recorded by the known methods. It may be apparent to one skilled in the art that embodiments of the method according to the invention using more than one trailing power level, such as a stepwise descent from the write power level to the erase power level, provide an even more gradual transition from the write power level of the last write pulse to the erase power level.
The method according to the invention has advantages especially when used in combination with a record carrier comprising an AgInSbTe alloy, such as, for example, a 4-layer stack of ZnS:SiO2/AgInSbTe/ZnS:SiO2/Al-alloy. These advantages can also be obtained when using record carriers comprising a GeInSbTe alloy. Experiments indicate that, when these alloys are used, the erase process of amorphous marks proceeds via growth of the surrounding crystalline edge to the center of the mark, rather than by nucleation and subsequent growth. Consequently, the written marks have a well-defined shape with sharp edges, leading to intrinsically lower jitter than observed in, for example, GeSbTe-based record carriers currently considered to be very promising for high speed recording. Moreover, the complete erasure time (CET) decreases rapidly with decreasing mark sizes. Because of this, the maximum data rate increases with a decreasing spot size of the radiation beam, which allows high recording speeds when going, for example, to lower wavelengths of the radiation source such as, for example, in the case of blue laser diodes having a wavelength at approximately 400 nm. Combining a record carrier comprising an AgInSbTe, or GeInSbTe, alloy with the method according to the invention allows recording marks to be obtained with a low jitter of the recorded marks at high recording speeds.
A preferred embodiment of the method according to the invention is characterized in that the last write pulse in a sequence of pulses has a second write power level (w2), the second write power level (w2) being higher than the first write power level (w1).
When the write power level of the last write pulse in a sequence of write pulses is increased compared to the write power level of the preceding write pulses in the sequence of write pulses, the trailing-edge jitter is even more reduced.
An embodiment of the method according to the invention is characterized in that the trailing power level (t) is dependent on properties of the record carrier, and a further embodiment of the method according to the invention is characterized in that the second write power level (w2) is dependent on properties of the record carrier.
The trailing power level (t) can be assigned a fixed chosen value. Alternatively, the trailing power level (t) can be assigned a value which value depends on properties of the individual record carrier where marks are to be recorded on. The value of the trailing power level (t) to be used for an individual record carrier can be determined, for example, by a test procedure where sequences of pulses, which sequence each have a different value for the trailing power level (t), are used to record marks, and the resulting marks are read back and are analyzed. Other test procedures can alternatively be used. Finally, the optimal value for the trailing power level (t) corresponding to an individual record carrier may be recorded on that record carrier. In this event the value can be directly read from the record carrier by a recording device. Likewise, the second write power (w2) can be assigned a fixed chosen value, or can be assigned a value which value depends on properties of the individual record carrier where marks are to be recorded on.
It is a further object of the invention to provide an optical recording device of the kind described in the second paragraph adapted for using the method according to the invention.
This object is achieved when the optical recording device of the preamble is characterized in that the trailing power level (t) is higher than the erase power level (e) and lower than the first write power level (w1).
A preferred embodiment of the recording device according to the invention is characterized in that the control unit controls the power of the radiation beam such that it has a second write power level (w2) during the last write pulse in a sequence of write pulses, the second write power level (w2) being higher than the first write power level (w1).